In recent years, there is an ever-increasing tendency towards high-density integration, high function, and high-density assembly of semiconductors extensively used, for example, in electronic equipments, communication instruments, and personal computers. This has led to a demand for better properties and higher reliability of laminated sheets for semiconductor plastic packages. In particular, there is a recent demand for a reduction in coefficient of thermal expansion in a plane direction of laminated sheets. When the difference in coefficient of thermal expansion between a semiconductor element and a printed wiring board for a semiconductor plastic package is large, warpage occurs in the semiconductor plastic package due to the difference in coefficient of thermal expansion upon exposure to thermal shock, sometimes leading to poor connection between the semiconductor element and the printed wiring board for a semiconductor plastic package or between the semiconductor plastic package and the printed wiring board mounted.
In addition, due to an increasing recent tendency toward an increase in density, high adhesion to copper foils for micro wiring formation and machinability in hole making by drilling or punching are required of copper-clad laminated sheets. Further, for electronic components, mounting by lead-free solder and imparting flame retardance without use of halogens have become required due to a growing interest in environmental problems, and, to meet this requirement, higher heat resistance and flame retardance than those of conventional products are required.
In order to meet the above requirements for a reduction in coefficient of thermal expansion, drilling workability, heat resistance, and flame retardance, metal hydrates are known as halogen-free flame retardants as an alternative to bromine-containing flame retardants. For example, aluminum hydroxide is known as a flame retardant that, when heated, causes a reaction that releases water of crystallization. When the amount of gibbsite that is a general structure of aluminum hydroxide is large, heat resistance is deteriorated due to an influence of water of crystallization that is released upon heating. The incorporation of boehmite obtained by hydrothermally treating aluminum hydroxide is also known (see, for example, patent document 1). Copper-clad laminated sheets prepared using boehmite are superior in heat resistance to those prepared using aluminum hydroxide, but on the other hand, drilling workability and coefficient of thermal expansion in a plane direction are unsatisfactory. Further, a mixture composed of hydromagnesite and huntite is known to improve flame retardance. However, only one example of the use of the mixture in thermoplastic resins such as polyolefin resins is described (see, for example, patent document 2), and the resin is not used for laminated sheet applications. Thus, the effect attained by this technique regarding a reduction in coefficient of thermal expansion and an improvement in heat resistance and drilling workability that are required of the laminated sheet applications has not been found.
A method in which an inorganic filler or an organic filler is filled into resins is known to reduce the coefficient of thermal expansion in a plane direction of laminated sheets (see, for example, patent document 3). This method suffers from a problem that increasing the amount of the inorganic filler filled into the resins to further lower the coefficient of thermal expansion is disadvantageous in that the resultant resin composition is hard and fragile, the abrasion of drill bits is fast, and breakage of drill bits and a lowering in the accuracy of hole position lead to an increased frequency of replacement of drill bits, resulting in a deteriorated drilling workability.